The present invention relates to improvements of a self-standing liquefied gas storage tank for a low temperature liquefied gas carrier ship which transports low temperature liquefied gas such as liquefied natural gas (LNG) and liquefied petroleum gas (LPG) or the like, and of a liquefied gas carrier ship having these tanks;.
Various types of low temperature liquefied gas carrier ships which transport low temperature liquefied gas such as LNG or LPG have been used. Some of these use self-standing liquefied gas storage tanks for transporting low temperature liquefied gas, and examples of these carriers are disclosed in Japanese Patent Application, First Publication No. 2-249796, Japanese Patent Aplication, First Publication No. 4-8999, and Japanese Patent Application, First Publication No. 4-92794.
An example of this carrier ship is shown in FIG. 12. This liquefied gas carrier ship may be easily have a flat deck 5 and makes it possible to increase the storage capacity of the liquid by forming liquefied gas storage tanks 4 carried in a ship's hull 1 into rectangular shades. Furthermore, it is contemplated, as shown in FIG. 13, to adopt a double casing structure (armored-structure) having an outer shell 1A and an inner shell 1B in the ship's hull 1 in order to ensure safety.
For such a low temperature liquefied gas carrier ship having rectangular-shaDed self-standing tanks, it is contemplated, as shown in an example in cross-sectional view in FIGS. 13 and 14, to mount the rectangular-shaped tank 4 to a ship's bottom 2 of the ship's hull 1, in a self-standing state, by a plurality of supporting devices 3. The tank 4 is constructed of alloy sheets such as aluminum alloy sheets. In FIG. 14, chain lines show that the tank 4 may contract, as indicated by continuous lines, when the tank 4 is filled with low temperature liquefied gas, and thus, the supporting devices 3 allow the contraction of the tank 4.
The portion above the tank 4 is covered by a deck 5, and the tank 4 is prevented from moving laterally by stops 6 provided between an under surface of the deck 5 and an upper surface of the tank 4.
However, the above structure requires the depth D of the ship's hull 1 to be increased, and this causes the weight of the ship and the amount of labor required to build the ship to increase. Furthermore, it is necessary to reinforce the deck 5 in order to restrain the lateral movement of the rectangular-shaped self-standing tanks 4. Additionally, since the upper surface of the ship's hull 1 is constituted by a single unitary integrated deck 5, stress in the deck 5 arising from the bending moment imparted by wave action is great. In this respect, the above structure again causes an increase in the weight of the ship and the amount of labor required.
An inner surface of a tank wall 7 of the rectangular-shaped self-standing tank 4 is provided with n%ain frames 8 and reinforcing plates 9 as stiffeners, as shown in FIGS. 13 and 15. The main frames 8 and the reinforcing plates 9 protrude from the inner surface of the tank wall 7 and are mutually transverse so that the entire tank can be made rigid. As shown in FIG. 15, each of the supporting devices 3 is located at positions corresponding to both the main frame 8 and the reinforcing plates 9. When the supporting device 3 is bearing the load of the tank 4, stress in the weld (fillet weld) portion between the inner surface of the tank wall 7 and the reinforcing plate 9 may increase.
As shown in FIG. 16, in between two adjacent reinforcing plates 9, a carling 11 is welded at. fillet weld portions 12 at a right angle to the plates 9 in order to reinforce the reinforcing plates 9 and fillet weld portions 10 thereof.
Although a decrease in the occurrence of stress near the fillet weld portions 10 may be anticipated by means of the carling 11, adequate examinations for stress in portions near the boundary section between a side surface of the reinforcing plate 9 and the upper end surface of the carling 11 are required.
if there is a gap G between the upper end surface of the carling 11 and a reinforcing face bar 9a, a welding torch can be inserted in the gap G, and this makes it possible to weld the portion 12 by boxing (box welding) so as to improve the strength of the reinforcing plate 9 and the fillet weld portions 10.
If the upper end surface of the carling 11 is directly welded to the lower face of the reinforcing face bar 9a, stress in the reinforcing plate 9 and the fillet weld portions 10 may be reduced. However, this structure makes it impossible to carry out the boxing (box welding), and thus reduces fatigue resistance.
Typically, a liquid storage tank or a shell of such a iquid carrier ship has a structuze in which a plurality of metal plates are transversely combined.
FIGS. 17-19 show reinforced structural bodies A, B, and C respectively which are conventionally used in such tanks, shells, or the like.
Referring to FIG. 17, the reinforced structural body A comprises a tank wall 7 of a bulkhead and a reinforcing member 14. The tank wall 7 is constituted by an integrated plate in which a plurality of plates are welded at butt weld joint portion 13. The reinforcing member 14 is welded on a surface of the tank wall 7 by fillet welding 15 transversely to the butt weld joint portion 13. A cut-out port 16 is formed on the reinforcing meraber 14 so as to prevent the butt weld joint portion 13 and the fillet weld portions 15 from interfering with each other. A li,zruid passing port 17 is also provided on the reinforcing member 14.
In the reinforced structural body B, as shown in FIG. 18, a reinforcing member 14 is welded on a surface of the tank wall 7 by fillet weld 15. Furthermore, a transverse reinforcing plate 18 is provided by fillet weld 19, in a standing state, on the same surface of the tank wall 7, transversely with respect to the reinforcing member 14. The transverse reinforcing plate 1St has a cut-out port 16 through which the reinforcing member 14 passes, and a tongue portion 18a protruding into the cut-out port 16. The protruding end of the tongue portion 18a is welded to one side of the reinforcing member 14 by fillet welding 20. In this case, the area adjacent to the fillet weld portions 15 is reinforced by the tongue portion 18a. The cut-out mort 16 doubles as a liquid passing port 17, and prevents the fillet weld portions 15, 19, and 20 from interfering with one another.
In the reinforced structural body B, as shown in FIG. 19, reinforcing members 14A, 14B, and 14C, which are parallel to one another, are welded to on a surface of the tank wall 7 by fillet weld portions 15, and ribs 21 are provided in between two of the adjacent reinforcing members. These ribs 21 are welded to both the tank wall 7 and the reinforcing members by fillet weld portions 22, 23, and 24. Cut-out ports 16 double as liquid passing ports 17, and prevent the fillet weld portions 15, 22, 23, and 24 from interfering with one another.
However, in the above-mentioned reinforced structures A, B, and C as shown in FIGS. 17-19, the fillet weld portions 19, 20, 22, 23, and 24 are discontinuous at the cut-out ports 16, and this obstructs a continuous weld. Thus this reduces the welding workability, or makes it difficult to use an automatic welding machine. Furthermore, the discontinuity of the welded portion causes stress concentration thereat, and leads to occurrences of imperfections, and therefore to a deterioration of welding quality an.d reliability.